田间小麦叶片光合效率日变化与光合“午睡”的关系

小麦灌浆初期叶片(旗叶)晴天中午光合速率下降(“午睡”)伴随了气孔导度、胞间CO_2浓度下降,而气孔限制值中午升高,进一步证实气孑L中午关闭是光合“午睡”的一个重要原因。叶片光合效率的中午下降并非都伴随着光合“午睡”现象。当两者同时发生时,胞间CO_2浓度降低,而光合速率与气孔导度、胞间CO_2浓度之间的相关性高于光合速率与光合效率之间的相关性。这些事实表明。即使光合效率中午下降是光合“午睡”的部分原因,但较之气孔中午关闭只是一个次要原因。

Relationship Between Diurnal Variations of Photosynthetic Efficiency and Midday Depression of Photosynthetic Rate in Wheat Leaves Under Field Conditions

We have reported that, under field conditions, photosynthetic rate of wheat flag leaves displays significant midday depression on bright sunny days in early summer (XU et al. 1984), and that on clear days photosynthetic efficiency in many C_3 plants, including wheat, often decreases around noon (XU et al. 1990). Naturally, the question arises whether the decreased efficiency is an important cause of the midday depression of photosynthetic rate. To answer this question, we made observations on the diurnal variation in apparent quantum yield (AQY), net photosynthetic rate (P_n), and stomatal conductance (G_S) for the flag leaves of wheat plants grown in pots using a portable ADC gas analysis system during early grain-filling stage. Also, based on gas exchange data, we calculated the intercellular CO_2 concentration (C_i) and stomatal limiting value (L_S), analyzed the correlaton between P_n and G_S, P_n and C_i, and P_n and AQY.It was found that midday depression of photosynthetic rate was accompanied by decreases in stomatal conductance and intercellular CO_2 concentration and by an increase in stomatal lim- lting value (Figs. 1,2). The results confirm the deduction presented in a previous paper that midday closure of stomata is an important cause of midday depression of photosynthesis. Midday decline of photosynthetic efficiency was not accompanied invariably by midday depression of photosynthetic rate (Fig. 5). Intercellular CO_2 concentratin decreased when midday depression of both the efficiency and the rate occurred simultaneously (Fig. 2). Furthermore , significant levels of correlation between photosynthetic rate and stomatal conductance as well as intercellular CO_2 concentration were higher than the correlation between photosynthetic rate and photosynthetic efficiency. These facts suggest that midday decline of photosynthetic efficiency may be, if at all, a less important cause for midday depression of photosynthetic rate than midday closure of stomata, at least under the conditions reported here.Midday decline of photosynthetic efficiency may be related to photoinhibition (Figs. 4, 3). Its mechanism needs further study.